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Old 12th February 2004, 09:36 PM   #1
amo is offline amo  United States
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Default Dual Transformers for a PS?

I have read that two diode bridges are better then one, and that this can be implemented using a transformer with dual seconderies. Obviously I do not know much about electronics, so I have to ask this basic question. How about using 2 transformers to power a gainclone? Sort of like in a BOSOZ..... One generating the + side and one generating the -. Would this be a better PS then one based on a single transformer? If this is possible and desirable, then how would it be wired? Sort of like in a BOSOZ????? Minus the Zenars? Just trying to complicate my life.....
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Old 12th February 2004, 09:51 PM   #2
GregGC is offline GregGC  Canada
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Default Re: Dual Transformers for a PS?

Quote:
Originally posted by amo
I have read that two diode bridges are better then one, and that this can be implemented using a transformer with dual seconderies. Obviously I do not know much about electronics, so I have to ask this basic question. How about using 2 transformers to power a gainclone? Sort of like in a BOSOZ..... One generating the + side and one generating the -. Would this be a better PS then one based on a single transformer? If this is possible and desirable, then how would it be wired? Sort of like in a BOSOZ????? Minus the Zenars? Just trying to complicate my life.....

Yes, you can do that (two transformers), but it's more expensive and it takes more space. As far as performance, it's the same.
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Old 12th February 2004, 09:59 PM   #3
sek is offline sek  Germany
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Hi,

Quote:
How about using 2 transformers to power a gainclone?
You mean for a single channel? Well, for two channels, that's probably true.

But I don't think that you'll gain much supply quality from using two transformers for one channel. They are more expensive, bigger in space requirements and different in parameters due to manufacturing tolerances.

OTOH, if you get them for a reasonable price, match them closely for identical output voltage (running both unloaded and under load) out of a bunch of available candidates, and have the mounting space available, then of course it would work.

But for use with a single (channel) gainclone you won't benefit much, as the resulting circuit would be equivalent to one with a single, bigger transformer.

To actually wire two separate transformers:

- wire the primaries of both transformers in parallel, just like wiring a single one, but with the second primary's wires exactly parallel to the first one's,

- use a single common primary fuse for both transformers (as well as a single mains switch), don't give them two dedicated fuses (as this would destroy your chip amp in case one of the fuses blows while the other doesn't),

- keep the transformer secondaries split for now, dedicate a rectifier full bridge (four diodes) to each secondary,

- connect the "-" output of the "upper" bridge to the "+" output of the "lower" bridge and name this signal your "power ground",

- use the "+" output of the "upper" bridge as your signal "V+",
use the "-" output of the "lower" bridge as your signal "V-".

In this scheme I assume that you intend to use transformers with a signle secondary winding (each). With transformers with two secondaries, your freedom to wire them grows "quad-fold", as well as your probability to fry them

edit:

Hey, Greg was a "little" faster while I was typing...
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Old 12th February 2004, 10:21 PM   #4
amo is offline amo  United States
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Sounds good to me! I hate that little voice inside my head that keeps telling me that there may be a better, harder way to get it done.... I think I understand now, however, that Mr. Pass used 2 transformers per channel in the BOSOZ because it is a ballanced circuit. Thanks!
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Old 17th February 2004, 02:34 PM   #5
Fishy is offline Fishy  Canada
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Smile Lowering the PS rail voltage?

I am trying to do exactly that as well ... since the transformer I got would only allow me to do a single secondary. HOWEVER, I have another couple of questions

(1) is it possible to make a (V+) - 0 - (V-) (i.e. center tapped) power supply out of single secondary?

(2) if the secondary output voltage is too high, what's an effective way of lowering it?

Thanks!
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Old 18th February 2004, 12:25 PM   #6
sek is offline sek  Germany
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Quote:
(1) is it possible to make a (V+) - 0 - (V-) (i.e. center tapped) power supply out of single secondary?
There are a couple of possible ways, but only one really works both safely and efficiently: unwinding the secondary, splitting it, rewinding the two halves in exactly the way the original wire was wound. Try that on a small, cheap transformer first!

Note that you reduce the power handling capability significantly, as now only half of the wire length will have to draw the same amount of current! Altogether, I wouldn't recommend to use a transformer that doesn't fit. I'd suggest you sell the one you have and buy a matching one.

Safety warning here: What I describe is fiddling around with mains components. Rewinding would be safe, but connecting it for a test run should be done with care.

Quote:
(2) if the secondary output voltage is too high, what's an effective way of lowering it?
That could be done by reducing the winding count of the transformer secondary. But as we already found out, you actually need a new transformer.
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Old 18th February 2004, 02:15 PM   #7
Fishy is offline Fishy  Canada
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Quote:
Originally posted by sek

That could be done by reducing the winding count of the transformer secondary. But as we already found out, you actually need a new transformer.
Thanks for your suggestion =) Certainly I don't wanna fiddle with the transformer windings at the moment

Is it also possible to use a voltage regulator, like the LM338 or LM318? I only need to lower the voltage from 50V DC to around 40V DC ...
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Old 18th February 2004, 04:53 PM   #8
sek is offline sek  Germany
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Quote:
Is it also possible to use a voltage regulator, like the LM338 or LM318?
I really don't recommend that, although it would work.

Quote:
I only need to lower the voltage from 50V DC to around 40V DC ...
I think you would be better off with a switching step-down converter due to better efficiency and less heat. Those are cheap (enough) and simple to implement. Now you just need someone to point you on a suitable regulator IC.

Sebastian.

PS: You have only one secondary but want to use the transformer to make a symmetric power supply, nevertheless?
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Old 18th February 2004, 05:19 PM   #9
GregGC is offline GregGC  Canada
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Quote:
Originally posted by Fishy


Thanks for your suggestion =) Certainly I don't wanna fiddle with the transformer windings at the moment

Is it also possible to use a voltage regulator, like the LM338 or LM318? I only need to lower the voltage from 50V DC to around 40V DC ...

LM338 will do fine if you want to get rid of 10V. You really have to use a good heat sink.

For 40W output power over 8Ohm sp. you'll have 2.3A. LM 338 would handle way more (8A max for 10V), so no worry. But again LM338 will dissipate 23W when the speaker's dissipating 40W so, you need a good heat sink and also insolated.

/Greg
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Old 18th February 2004, 05:41 PM   #10
Fishy is offline Fishy  Canada
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Wow ... thanks for all those info ...

Well, I will use two separate toroids to power the LM3875. One for +ve, and the other one for -ve. It just happened that the toroids I got have too high a secondary voltage for the LM3875. That's why I am asking all those questions about lowering the voltage and such.

I thought about the LM338 for the positive side of things ... however, I cannot find the -ve counterpart of the regulator that has the same current capabilities (the LM 317 can only do 1.5A)

I will do some research on the switching step-down converter ... hope it would work out fine I really don't want the step down circuit to eat up too much power ...
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